1. Field of the Invention
The present invention relates to an image reading apparatus for reading thick documents such as books and the like, and specifically relates to art for detecting the height of a document such as a book and correcting image distortion in accordance with the detected document height, and art for detecting changes in background luminance arising from the curvature of a document when a document such as a book is placed open-faced on a document platen, and automatically correcting the density of the reproduced image in accordance with the detected changes in background luminance of the document.
2. Description of the Related Art
In conventional image reading apparatuses of this type, the shape of the portion corresponding to the head of a book document is read by a charge-coupled device (CCD) sensor or the like to measure the height distribution of a book document placed on a document platen in an open-faced state. When a stamp is pressed on the head of the book-like document to adhere attachments such as a label or the like, a problem arises in that the document height distribution cannot be obtained to correct for image distortion.
Conventional art is known to automatically correct the density of the reproduced image by detecting change in the .background luminance arising from the curvature of the book-like document and effecting said correction in accordance with the detected change in background luminance of the book-like document. When a photograph or the like is present in a part of the imaged page, however, the background luminance cannot be accurately measured, such that image density cannot be properly controlled.